Bone anchors may be used in orthopedic surgery to fix bone during healing or during a fusion process. In spinal surgery, bone anchors may be used with spinal fixation elements, such as spinal rods, to stabilize multiple vertebrae either rigidly, in which no relative motion between the vertebrae is desired, or dynamically, in which limited, controlled motion between the vertebrae is desired. Fixation elements can help to support the spine in a desired alignment, for example by defining a shape towards which a deformed spine is to be corrected. Attaching the vertebrae to the fixation element causes vertebrae which are out of position to be drawn towards the fixation element, so that they can then be retained in a correct alignment against forces imposed by soft tissue tending to revert the configuration of the spine towards the deformed shape. Correction of the spinal deformation can involve application to the vertebrae of translational forces, torsional forces, or combinations thereof to cause vertebrae to translate and/or rotate.
Surgical procedures using bone anchors often require that the bone anchor be secured to the bone at a predetermined angle to a surface of the bone. Traditional bone anchors can include a shaft having a spherical head that is polyaxially seated in a rod-receiving member and that can be secured at a fixed angle to the rod-receiving member by a compression member disposed proximally of the head. Traditional compression members have distal surfaces that are formed as a negative of the head and exert a distal force on the head to maintain the shaft at the predetermined angle. However, where the shaft is positioned at an angle to a longitudinal axis of the compression member, a contact surface area between the compression member and the head can be reduced, which can increase a risk of slippage between the bone anchor and the compression member. Slippage between the bone anchor and the compression member can cause the bone anchor to move from a desired angle and can therefore compromise the effectiveness of the bone anchor for correcting spinal deformities.
Accordingly, there remains a need for improved methods and devices for bone anchor fixation.